1. Field of the Invention
This invention relates to a manual hoisting apparatus of the lever type or chain block type. More particularly, this invention relates to a manual hoisting apparatus such that the rotation in the hoisting or winding direction is automatically precluded under overload and that even when the apparatus is overloaded by an external force acting after hoisting or pinching of a load, the rotation of a drive ring is positively transmitted to a drive shaft to lower or release the load.
2. Description of the Prior Art
The conventional manual hoisting apparatus includes the one proposed by the present applicant and disclosed in Japanese Unexamined Patent Publication No. 60-202093.
The apparatus described in the above patent literature comprises a load sheeve rotatably supported by side plates constituting the body of the hoisting device and drive shaft rotatably mounted on said load sheave. This drive shaft is extending beyond the side plates supporting the load sheave. The portion of said drive shaft which extends from one of said plates is formed with a thread. Screwed onto this thread is a shaft driving member, to which a force exerting member is further threaded. Secured to the other projecting end of said drive shaft is a drive pinion which is arranged to drive the load sheave via a reduction gear train. The shaft driving member mentioned above has a boss extending towards said force exerting member. Rotatably mounted on said boss is a ratchet wheel which is flanked by friction disks. The ratchet wheel is engaged with an anti-reversal ratchet pawl in such a manner that it is rotatable only in the winding or hoisting direction. It should be understood that reference to the winding or hoisting direction refers to the direction in which any member rotates when sheave 3 is rotating in a winding or hoisting operation; conversely, reference to the unwinding direction of movement means the direction that any member rotates when the sheave 3 is unwinding for lowering its load.
Further, the above force exerting member carries a manual chain sprocket which is pressed by a conical friction ring at a predetermined pressure.
Thrown on said chain sprocket wheel is a manipulating chain and the load sheave is turned in the hoisting direction or the lowering direction via said sprocket wheel.
In the above conventional hoisting apparatus, if one attempts to hoist up a load under overload conditions, the torque applied to the ratchet wheel through said sprocket wheel, force exerting member and friction disk upon pulling of the chain is exceeded by the force pressing the force exerting member against the shaft driving member via said drive shaft owing to the overload, with the result that said sprocket wheel idles on the conical friction ring. Therefore, the hoisting of the overload in excess of the rating is automatically arrested.
To lower the suspended load, the sprocket wheel is turned in the unwinding direction to reduce the pressing force acting on the friction disk and to thereby rotate the drive shaft in the unwinding direction.
In the above arrangement, however, in the situation where the hoisting apparatus is used for pinching the truck load and the operation is carried out within the rating, the load sheave may be subjected to an unexpected overload owing to the vibrations of the truck or a sudden displacement of the truck load or owing to an external force that may act on the suspended load. In such an event, the force applied by the load sheave to rotate the drive shaft causes the force exerting member to be pressed hard against the friction disk.
Therefore, even if the sprocket wheel is turned in the unwinding direction to drive the load sheave in the unwinding or loosening direction, the torque exerted by said friction disk to arrest rotation of the force exerting member exceeds the torque applied to said sprocket wheel, with the result that the sprocket wheel idles with respect to the conical friction ring, thus preventing the unwinding (loosening) rotation of the load sheave. Therefore, once such a situation develops, it is time-consuming and troublesome for the operator to take the load off.